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Abstract-Features of oogenesis, ovarian maturity phases, and fecundity of the smooth lumpsucker (Aptocyclus ventricosus) are described here for the first time. Gonadal development was examined by histological methods and the morphological features and size of germ cells at various stages was determined. These features were used to more accurately define morphological criteria of 7 histological and macroscopic maturity phases. The smooth lumpsucker is an iteroparous gonochoristic species with determinate fecundity, group-synchronous ovary organization, total spawning (release of 1 batch of eggs per breeding season), and external fertilization.

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The first description of the histology of the digestive tract of the lumpfish Cyclopterus lumpus showed an adaptation for collecting and storing water. The morphology of this species' tract is mostly typical of carnivorous fishes, having a sac‐like stomach for storage and a relatively short intestine of approximately 1.2× the standard body length. The esophagus is relatively short (0.13× standard body length) and thick‐walled; it has a thick muscular coat, and its thickness changes from 0.4 to 1 mm during contraction. The stomach has well‐developed gastric glands and is divided into the histologically distinguishable cardiac, fundus, and pyloric parts. The intestine has the same structure throughout its entire length. The water‐holding ability of lumpfish is provided by well‐developed muscles in the esophagus and stomach distensibility, which differs morphologically from the method used by Tetradontidae and Diodontidae species, who store water in the caudal part of the esophagus. The morphology of the digestive tract proved that this species is an intermediate between Tetradontidae and Diodontidae, which could inflate their bodies by swallowing certain amounts of water, and other fishes which are not able to do so. Results of the work extend current knowledge on the morphological capability of the digestive system of teleost fish, providing water‐blowing behavior.